Structure and function of shisham forests in central Himalaya,India: nutrient dynamics

The structure and function of Shisham (Dalbergia sissoo Roxb.) forests were investigated in relation to nutrient dynamics in 5- to 15-year-old stands growing in central Himalaya. Nutrient concentrations and storage in different layers of vegetation were in the order: tree > shrub > herb. Forest soil, litter and vegetation accounted for 80.1-91.9, 1.0-1.5 and 7.0-18.4%, respectively, of the total nutrients in the system. There were considerable reductions (trees 32.8-43.1; shrubs 26.2-32.4; and herbs 18-8-22-2%) in nutrient concentrations of leaves during senescence. Nutrient uptake by the vegetation as a whole and also by the different components, with and without adjustment for internal recycling, was investigated. Annual transfer of litter nutrients to the soil from vegetation was 74.8-108.4 kg ha(-1) year(-1) N, 56.8-4 kg ha(-1) year(-1) P and 38.7-46.9 kg ha(-1) year(-1) K. Turnover rate and time for different nutrients ranged between 56 and 66 % year(-1) and 1.5 and 1.8 years, respectively. The turnover rate of litter indicates that over 50% of nutrients in litter on the forest floor are released, which ultimately enhances the productivity of the forest stand. The nutrient use efficiency in Shisham forests ranged from 136 to 143 kg ha(-1) year(-1) for N, 1,441 to 1,570 kg ha(-1) year(-1) for P and 305 to 311 kg ha(-1) year(-1) for K. Compared with natural oak forest (265 kg ha(-1) year(-1) and an exotic eucalypt plantation (18 kg ha(-1) year(-1), a higher proportion of nutrients was retranslocated in Shisham forests, largely because of higher leaf tissue nutrient concentrations. This indicates a lower nutrient use efficiency of Shisham compared with eucalypt and oak. Compartment models for nutrient dynamics have been developed to represent the distribution of nutrients pools and net annual fluxes within the system.